Call me crazy, but if I were doing Age of Ultron, I would make Ultron a threat that the main team, not including Pietro and Wanda, can't handle alone. And I would, in turn, introduce a team of "New Avengers" consisting of Pietro, Wanda, Winter Soldier, Falcon, and War Machine (As well as, in a perfect world, Spidey ).

No one other than Pietro and Wanda would appear outside of the third act, and they would really serve as a reminder that the team of Avengers from the first film aren't the only heroes out there who could, and would, help in a global crisis such as one Ultron will cause, and it would serve as a precedent for an even wider spread gathering of heroes than that in Avengers 1 that will be needed in the future (the kind that will be needed to stop Thanos).